In place of Stilton, you could use a creamy blue cheese, like Gorgonzola dolce, which is a little less assertive.
- 4 pounds sweet potatoes (about 8–10 medium), peeled, sliced on a diagonal ½” thick
- Kosher salt and freshly ground black pepper
- 1 large shallot, finely chopped
- 2 tablespoons white wine vinegar
- 2 tablespoons fresh lemon juice
- 2 tablespoons walnut oil or olive oil
- Vegetable oil (for frying; about ½ cup)
- 4 oz. Stilton cheese, shaved
Preheat oven to 400°. Mix sugar and 1 Tbsp. hot water in a small bowl until sugar dissolves. Add walnuts and toss to coat. Spread out on a baking sheet. Toast, tossing halfway through, until golden brown, 18–20 minutes. Let cool then break into pieces (sugar will harden as nuts cool).
Toss sweet potatoes and olive oil on a parchment-lined baking sheet; season with salt and pepper. Arrange in a single layer and roast until tender and golden brown, 25–30 minutes. Let cool.
Combine shallot and vinegar in a medium bowl and let stand 15 minutes. Whisk in lemon juice and walnut oil; season with salt and pepper.
Meanwhile, pour vegetable oil into a small skillet to measure ½” and heat over medium heat. Add sage leaves and fry until crisp, 30–40 seconds. Using a slotted spoon, transfer sage leaves to a paper towel–lined plate; set aside.
Place sweet potatoes on a platter; drizzle with vinaigrette and top with blue cheese, walnuts, and fried sage.
DO AHEAD: Nuts can be candied 1 day ahead; store airtight at room temperature. Vinaigrette can be made 1 day ahead; cover and chill.